This invention relates to a heat recovery apparatus and method to recover heat from a fluid bed chemical reactor. In particular, the invention relates to the lowering of the temperature of the reaction product outflow from a fluidized reactor bed.
A wide variety of chemical processes are conducted in fluid beds. Included in such processes are chemical reactions, calcination and absorption. Examples of such processes are oxychlorination of ethylene with hydrogen chloride and air to produce ethylene dichloride, roasting of pyrites with air to produce sulfur dioxide, drying and/or calcining of phosphate rock, production and reduction of uranium hexafluoride, hydrofluorination, fluorination and oxidation of o-xylene to phthallic acid.
Conventionally, a reactor chamber is provided at the lower end with one or more inlets through which gaseous reactants, carriers and/or processing gaseous streams are introduced. A particulate material is supported by a grate or other support in the path of the gas flow. At sufficient gas velocity, the fluidization begins and a fluid bed is formed above the grate. Product gas flow from the fluid bed passes downstream, usually upward, frequently through a cyclone in which carried particles or elutriates are recovered.
In many such processes the fluid bed operates at a sufficiently high temperature, either because of preheating of the gases or because of an exothermic reaction, to recover heat in the reaction fluid bed. Such temperatures exceed 500.degree. C. and frequently exceed 700.degree. C. Devices are also sometimes provided outside the reactor to recover heat from the effluent, as in a waste-heat boiler. In such large boilers, high pressure steam over about 200 psi and generally over about 450 psi is generated. In many systems, the gas outflow passes through a heat exchanger in a countercurrent relation to a coolant, such as water. An economizer is sometimes used to preheat the water with the gaseous outflow from the waste heat boiler.
A waste heat boiler has distinct disadvantages with such systems. When an undesired side or back reaction can occur in the effluent, it is usually desirable to quickly quench the effluent, which is not done by a waste heat boiler. Secondly, many reaction product gases include highly corrosive components, particularly water and acids such as hydrochloric acid and hydrogen fluoride. Such components are frequently most corrosive at the elevated reaction temperatures, and will therefore attack metal and other surfaces in the cyclone or inlet portions to the waste heat boiler. Furthermore, the large waste heat boiler used is usually not designed to withstand the high pressures of some fluid bed reaction systems or to operate economically with many fluid bed reaction systems.
Moreover, there are many reactions in which the reaction gas outflow is below about 700.degree. C., and particularly below about 500.degree. C., and thus no high pressure steam could be economically recovered. Especially when undesired side or back reactions occur in the reaction product gas at reaction temperatures, the reaction product gas is usually quenched in ways that heat is not recovered in usable form.
Additionally, in reaction product gases containing both acids and water, it is most undesirable that the product gas cool to below the dew point of the water vapor. When this occurs, an aqueous acid solution is formed which may be more corrosive than the gaseous water vapor and acid. Most quenching systems have inadequate temperature control to prevent this condensation.
Finally, waste heat boilers do nothing to assist the cyclone in the recovery of particles from the fluid bed carried by the gas outflow. Instead, such materials as copper catalysts which begin to form a vapor at about 400.degree. C. may travel past the cyclone and cause potential corrosion, contamination or pollution problems downstream from the cyclone.
It is known to recover heat from within a reaction bed in which an exothermic reaction is occurring. Materials such as liquid salts are conventionally passed in a heat exchanger tube or coil through the reaction fluid bed to recover heat at the reaction temperature and prevent the reaction bed from overheating. If the salt absorbs the heat at temperatures above about 350.degree. C., high temperature steam may be generated outside the reactor by heat exchange contact with the liquid salt.
It is also known to provide chambers with multiple fluid beds. Particularly for calcining processes, the particulate material to be calcined is conventionally passed downward through a series of fluid beds being successively heated by a heating gas flow passing upward through the same fluid bed. A similar arrangement is provided in systems in which a solid particle, such as alumina, is reacted with a gaseous reactant, such as hydrogen fluoride, to produce a solid product, such as aluminum fluoride. An example of such a process is disclosed in U.S. Pat. No. 3,473,887.
U.S. Pat. No. 3,967,975 discloses a housing with an upper and lower chamber and series of connecting tubes therebetween, with a fluidizing gas fluidizing material in both chambers and the connecting tube. Heat transfer jackets are provided for each chamber and the tubes. The material forms a continuous fluidized bed in the tubes in the adjacent portions of each chamber, preventing the maintenance of any temperature differential therebetween.
U.S. Pat. No. 3,795,490 discloses an apparatus for thermal cracking of hydrocarbons including a lower heating and reaction tower (in which heated molten metal circulates), an intermediate transfer line portion and an upper quenching tower with quenching tubes. Fluidized sand particles occupy the intermediate transfer line portion, the adjacent portion of a lower tower down to the molten metal surface and the adjacent portion of the quenching tower up to above the quenching tubes. Although a temperature differential must be maintained between the molten metal and the quenching tubes, the continuous fluidized bed of sand particles would tend to conduct heat therebetween and increase the amount of heat input required for the molten catalyst, while reducing the effectiveness of the quenching tubes.
U.S. Pat. No. 3,307,921 discloses a reactor with multiple reaction-promoting catalyst beds in series and between each bed a heat exchanger section to withdraw heat from the reactor fluid as it passes from one stage to the next. In such a device, the only heat exchange surfaces are those of the heat exchanger, and the relatively large heat exchange surfaces of the particles are not utilized.
U.S. Pat. No. 2,779,777 discloses cooling coils immersed in particle beds. U.S. Pat. Nos. 2,622,970 and 2,926,143 disclose such coils immersed in a fluid bed of catalytic particles.
In none of the references is the high conductivity of fluidized particles utilized to quickly withdraw heat from the effluent from a hot reaction zone without simultaneously withdrawing heat from the reaction zone itself. The temperature differential required for efficient, rapid quenching of the reaction effluent and good waste heat utilization cannot be maintained if the fluidized particles can rapidly transfer heat out of the reaction bed into the zone through which the effluent must pass.